The optimal energy intake of growing premature infants has not been determined. Moreover, information regarding the influence of energy and protein intake on the rate and composition of weight gain are not available. There are several reasons for this dearth of information: controlled studies comparing feeding regimens are difficult to perform in small premature infants; past investigators have focused on weight gain but have not assessed concurrent changes in body composition; the problem is confounded by the complex interaction of energy and protein intakes and their effects on growth. Animal studies and a preliminary study in premature human infants suggested that variations in energy intake might influence the composition of weight gain when protein intake is controlled. WE plan o compre two levels of energy intake in infants with birth weight below 1.5 kg who have begun to gain weight. Two groups of intants will be fed equal volumes of two formulas with the sameprtein concentration (2.2 g/dl) but different energy densities (83 vs. 72 kcal/dl). When receiving 150 ml/kg per day both groups will receive 3.3 g protein/kg per day. The energy intakes will be 124 and 108 kcal/kg per day. A preliminary study found similar rate of gain in both groups receiving these intakes of proten and energy, suggesting difference in the composition of gain. We will measure body weight, length, head circumference and skinfold thickness serially and will measure energy expenditure (indirect calorimetry), energy intake and excretion (bomb calorimetry) total body water (deuterium oxide space), and extracellular water (corrected bromide space), of each infant twice: at or near the time when enteral intake is established and growth is beginning (age 7 to 14 days), and again three weeks later. By measuring these changes concurrently we will be able to determine the effect of this difference in energy intake (and protein-to-energy ratio) on the rate and composition of weight gain and the energy cost of growth. We hypothesize that the higher energy intake will lead to greater fat content of weight gain and consequently greater energy cost of growth.